improve: error message when rendering XML example (via #5253)

The original message gave no clues as to the underlying cause, which is that the element name needs to be specified explicitly (using xml/name) if it can't be determined from a $ref.

src/core/utils.js

@@ -632,7 +632,7 @@ export const getSampleSchema = (schema, contentType="", config={}) => {
         let match = schema.$$ref.match(/\S*\/(\S+)$/)
         schema.xml.name = match[1]
       } else if (schema.type || schema.items || schema.properties || schema.additionalProperties) {
-        return "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<!-- XML example cannot be generated -->"
+        return "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<!-- XML example cannot be generated; root element name is undefined -->"
       } else {
         return null
       }
@@ -842,4 +842,4 @@ export function paramToValue(param, paramValues) {
     .filter(value => value !== undefined)
 
   return values[0]
-}
+}